She said: Based on the positive identification of this chemical agent by world-leading experts at the Defence Science and Technology Laboratory at Porton Down; our knowledge that Russia has previously produced this agent and would still be capable of doing so; Russia’s record of conducting state-sponsored assassinations; and our assessment that Russia views some defectors as legitimate targets for assassinations; the Government has concluded that it is highly likely that Russia was responsible for the act against Sergei and Yulia Skripal.

Mrs May said the substance must either have been used in a state attack, or have got into the hands of an individual, in which case Russia had lost control of the agent.

She added: The Foreign Secretary has summoned the Russian Ambassador to the Foreign and Commonwealth Office and asked him to explain which of these two possibilities it is – and therefore to account for how this Russian-produced nerve agent could have been deployed in Salisbury against Mr Skripal and his daughter.

My Right Honourable Friend has stated to the Ambassador that the Russian Federation must immediately provide full and complete disclosure of the Novichok programme to the Organisation for the Prohibition of Chemical Weapons.

And he has requested the Russian Government’s response by the end of tomorrow.

Tensions are high in Salisbury, where many feel they are not being given enough information quickly enough to understand what is going on. Officials have said there is low concern for public health.

Alexey Navalny, leader of the opposition in Russia, who is banned from standing against Vladimir Putin, tweeted: What British sanctions in response to the (attempted) murder of Skripalja (if Russia’s involvement is proved) would be very effective and supported by public opinion within Russia?

The answer consists of three surnames: Abramovich, Usmanov, Shuvalov.

The White House responded by calling the attack an outrage, while Downing Street revealed Mrs May had spoken to Emmanuel Macron on Monday night, and he condemned the attack.

US Secretary of State Rex Tillerson said: We have full confidence in the UK’s investigation and its assessment that Russia was likely responsible for the nerve agent attack that took place in Salisbury last week.

Those responsible – both those who committed the crime and those who ordered it – must face appropriately serious consequences.

We stand in solidarity with our allies in the United Kingdom and will continue to co-ordinate closely our responses.

NATO Secretary General Jens Stoltenberg said: The use of any nerve agent is horrendous and completely unacceptable.

The UK is a highly valued ally, and this incident is of great concern to NATO.